Enable SSH on a headless Raspberry Pi (add file to SD card on another machine)
For headless setup, SSH can be enabled by placing a file named ssh, without
any extension, onto the boot partition of the SD card from another computer.
When the Pi boots, it looks for the ssh file. If it is found, SSH is enabled
and the file is deleted.
The content of the file does not matter; it could contain text, or nothing
at all.